1. Field of the Invention
The present disclosure relates to methods for producing spin injection electrodes. More specifically, the present disclosure relates to a method for producing a spin injection electrode for injecting spins into graphene.
2. Description of Related Art
Substances made of carbon (C) take a wide variety of forms including, as well as diamonds, sheets, nanotubes, horns, and balls such as C60 fullerene. Furthermore, the physical properties of such substances are more various than their forms. The rich variety of physical properties prompts energetic research and development for application of the substances. Among such substances, a carbon thin film composed of one or several layers is called graphene. Graphene is a substance the isolation of which was realized in 2004, and its singular physical properties as two-dimensional semimetal have been discovered one after another (Science, vol. 306, pp. 666-669 (2004)). For example, graphene exhibits an electron mobility which is ten times or more that of silicon (Si). Therefore, the use of graphene has the possibility of realizing a high-speed and low-consumption electronic device (JP 2009-182173 A). In graphene, spin orbit interaction, which acts as a major factor of spin scattering when spins are used as carriers, is extremely small. For this reason, graphene is expected to be utilized for spin devices (Advanced Functional Materials, vol. 19, pp. 3711-3716 (2009)).
A ferromagnetic material is used as an electrode for spin injection into a graphene. The ferromagnetic material is spin polarized since there is a difference between the numbers of electrons with up-spins and with down-spins contained in the material. Accordingly, this difference is reflected also in spins injected into the graphene, and the graphene can be utilized as a spin device. The graphene used as a spin device may be a single-layer graphene or a multi-layer graphene having two or more layers.
High-efficiency spin injection into a graphene is required in order to achieve sufficient performance as a spin device. The efficiency PN in spin injection is represented by the following expression: PN=PF/[1+(1−PF2)·(RN/RF)](OYO BUTURI (Applied Physics), vol. 77, pp. 255-263 (2008)). In this expression, PF is a spin polarization ratio of a ferromagnetic material, RF is a resistance of the ferromagnetic material, and RN is a resistance of a non-magnetic material. According to this expression, if the non-magnetic material has a much higher resistance than the ferromagnetic material, i.e., in the case of RN>>RF, a relation of PN<<PF is derived, and this means the spin polarization is remarkably reduced at the interface between the materials. Therefore, spins cannot be injected into the non-magnetic material sufficiently. The same applies to the case where the non-magnetic material is a graphene. For good spin injection into a graphene, it is important to ensure consistency of the interfacial resistance between the graphene and a ferromagnetic material which functions as a spin electrode. At present, however, the efficiency in spin injection from a ferromagnetic material into a graphene is extremely low (Journal of the Surface Science Society of Japan, vol. 31, pp. 162-168 (2010)).
In order to realize high-efficiency spin injection into a graphene, the following approaches can be conceived: (1) resolution of inconsistency of the interfacial resistance between a ferromagnetic material and a graphene; and (2) use of a ferromagnetic material that exhibits a high spin polarization ratio PF. For the approach (1), OYO BUTURI, vol. 77, pp. 255-263 (2008) discloses a method in which a barrier layer such as a tunnel barrier is provided between a ferromagnetic material and a graphene. Nature, vol. 448, pp. 571-575 (2007) discloses an example in which aluminum oxide is used as a barrier layer, and Physical Review B, vol. 77, 020402(R) (2008) discloses an example in which magnesium oxide is used as a barrier layer. These barrier layers are each formed on a graphene in advance before a ferromagnetic material is deposited on the graphene. According to these documents, a device structure that allows spin injection from the ferromagnetic material into the graphene is built through the two steps, i.e., the step of forming a barrier layer on the graphene in advance, and the step of depositing the ferromagnetic material on the formed barrier layer. However, in the example disclosed in Nature, vol. 448, pp. 571-575 (2007), the resistance of the device is high, and a good relationship is not established between the interfacial resistance and the spin injection. Therefore, the spin injection efficiency in this example is low, and is 0.1 at a room temperature. In the example disclosed in Physical Review B, vol. 77, 020402(R) (2008), the spin injection itself is not realized at a room temperature. According to determination based on band calculation, ferromagnetic materials that exhibit a high spin polarization ratio PF include Heusler alloys, CrO2, and Fe3O4 which is an iron oxide (Journal of the Physical Society of Japan, vol. 68, pp. 1607-1613 (1999)). In band calculation, these ferromagnetic materials exhibit perfect spin polarization. However, although these ferromagnetic materials need to include crystalline phase in order to obtain a high spin polarization ratio, the crystalline phase of each of these materials is not lattice-matched to a graphene having a two-dimensional planar structure composed of six-membered rings. For this reason, spin injection electrodes that use these ferromagnetic materials have not been reported thus far. A method that can realize high-efficiency spin injection into a graphene has been desired.